The invention relates to a variable magnification type copying machine with a zoom lens. More particularly, the invention relates to the structure of a zoom lens barrel with two movable lenses for use in such a copying machine.
FIG. 1 shows an example of the loci traced by a front lens group L1 and a rear lens group L2, forming a copying zoom lens L, in moving between magnification settings. In this case, an original surface (original platen) D and a photosensitive surfaces S are fixed, that is, the distance between the object and its image remain unchanged.
FIGS. 2 through 4 show an example of the construction of a conventional variable magnification type copying machine in which the front lens group L1 and the rear lens group L2 are moved along the above-descried loci. In FIG. 2, an original platen D and a zoom lens L, composed of a front lens group L1 and a rear lens group L2, are the same as those shown in FIG. 1, and a photosensitive surface S is the surface of a photosensitive drum. The original platen D is a transparent plate such as a transparent glass plate. A "full-speed" mirror M1, which moves together with an illuminating lamp R1, is provided below the original platen D. A light beam reflected from the full-speed mirror M1 is received by optical path changing mirrors M2 and M3 which are supported so as to be movable as a single unit. In order to maintain the optical path length unchanged, the mirrors M2 and M3 are moved in the same direction as the full-speed mirror M1 but at half the speed of the full-speed mirror M1. Therefore, the mirrors M2 and M3 are sometimes called "half-speed" mirrors. The direction of advance of the light beam reflected by the full-speed mirror M1 is changed 180.degree. by the optical path changing mirrors M2 and M3 so that the light beam is applied through the zoom lens L to a fourth mirror M4. The light beam is reflected by the fourth mirror M4 so that it is applied to the photosensitive drum S, which rotates in synchronization with the full-speed mirror M1 (speed of full-speed mirror M1=(1/magnification).times. peripheral speed of drum S). In order to change the magnification, the movable lens groups L1 and L2 of the zoom lens L are moved along the optical axis, and the distance between these lens groups is set as described with reference to FIG. 1.
FIGS. 3 and 4 shows an example of a conventional mechanism for moving the movable lens groups L1 and L2. In the lens group moving mechanism, the force of movement of the front lens group L1 is used to move the rear lens group L2 with the aid of a cam. In these figures, reference character F designates the optical axis of the zoom lens L, and 11 a guide bar secured to supporting brackets 13 on a base 12 and mounted parallel to the optical axis F. A zoom lens frame 14 composed of a first frame 14A and a second frame 14B is slidably mounted on the guide bar 11. Guide rollers 15 are provided at the end of the zoom lens frame 14 which roll on a guide rail 16 fixedly provided on the base 12 and extending parallel to the guide bar 11.
The front lens group L1 is supported by a first lens barrel 17 which is screwed into the first frame 14A so as to be fixedly secured thereto, as shown in FIG. 4. The rear lens group L2 is supported by a second lens barrel 18 which, as in the case of the first lens barrel 17, is screwed into the second frame 14B so as to be fixedly secured thereto. The first and second frames 14A and 14B are movably mounted on the guide bar 11 in such a manner that the second frame 14B is movable with respect to the first frame 14A
A feed screw 22 is supported by the supporting brackets 13 parallel to the guide bar 11. The feed screw 22 thus supported is threadably engaged with an arm 21 which is extended from one end of the first frame 14A. The feed screw 22 is driven by a stepping motor (or a pulse motor) 23, whose rotational position is controllable, via a timing pulley 24, a timing belt 25 and a timing pulley 26.
A shaft 28 is supported by the first frame 14 perpendicular to the optical axis F. A distance adjusting member, namely a cam plate, is rotatably mounted on the shaft 28. A cam follower 30 adapted to abut against the peripheral cam surface of the cam plate 29 extends from the second frame 14B to which the second lens barrel 18 has been secured as described above. The cam plate 29 and the cam follower 30 are maintained in elastic contact with each other by a tension spring 31. Therefore, as the cam plate 29 rotates, the second frame 14B, and accordingly the second lens barrel 18, is moved along the optical axis F along a locus determined according to the configuration of the cam surface of the cam plate 29. The cam plate 29 is shaped so that the rear lens group L2 supported by the second lens barrel 18 is moved along the locus G shown in FIG. 1.
The cam plate 29 is coupled via a shaft to a wire drive pulley 33. The wire drive pulley can be turned relative to the shaft for phase adjustment, and it is fixed to the shaft after the phase adjustment has been accomplished. A flexible wire 35, fixedly secured at a center point to the wire drive pulley 33 with a small screw or the like, is wound on the wire pulley 33. The opposed ends of the wire 35 are fastened to respective ones of the supporting brackets 13.
In the conventional device thus constructed, as the feed screw 22 is rotated by the stepping motor 23, the first frame 14A is moved along the guide bar 11, and the zoom lens L composed of the front lens group L1 and the rear lens group L2 is moved, in its entirety, along the optical axis F. In this operation, according to the magnification selected, the first frame 14A follows a locus equal to the locus E of the front lens group L1 in FIG. 1.
The middle part of the wire 35 is fixedly secured to the wire drive pulley 33 and its ends thereof are fixedly fastened to the supporting brackets, as described above. Therefore, as the first frame 14A is moved, the wire drive pulley 33, and accordingly cam plate 29 (which is mounted on the same shaft as the wire pulley), are turned through an angle corresponding to the amount of movement of the first frame 14A. Thus, the second lens barrel 18 (and hence the second frame 14B) is moved according to the configuration of the cam plate 29 with the aid of the cam follower 30, which is elastically abutted against the cam plate 29, so that the distance between the front lens group L1 and the rear lens group L2 is set to a value corresponding to the desired magnification, that is, the rear lens group L2 is moved along the curve G in FIG. 1.
In the conventional machine, the mechanism for supporting and adjusting the front and rear lens groups L1 and L2 forming the zoom lens L suffers from a drawback in that the first and second lens barrels 17 and 18 are screwed into the first and second frame 14A and 14B to be secured thereto, as described above. In this connection, in order to prevent the lenses thus held from being inclined, the threaded parts should be relatively long. Therefore, it takes a relatively long period of time to machine and assemble those parts.
Moreover, the initial position of the zoom lens in the direction of the optical axis is adjusted by turning the first lens barrel 17 and the second lens barrel 18. However, they cannot be turned under the condition that the position in the direction of the optical axis has been determined, and therefore, the best part of the performance of the zoom lens cannot be effectively used. This will be described in more detail.
In the above-described variable magnification type copying machine, as shown in FIG. 5, a light beam reflected from the original surface D is applied through the lens groups L1 and L2 to the photosensitive drum S where it forms a slit-shaped image. Therefore, the lens groups L1 and L2 need provide their optimum performance in a particular direction, that is, only for the slit-shaped image formed on the photosensitive drum S. Ideally, a lens should form a completely uniform circular image, but it is difficult to manufacture a lens which is uniform in performance for all parts of the circular image because errors are generally involved in its manufacture. In other words, the best part of the performance of the lens for the slit-shaped image can be found by turning the lens. However, with the conventional approach for adjusting the position of the lens in the direction of the optical axis, this cannot be done without changing the lens's position in the direction of the optical axis. Accordingly, it is necessary to use a lens which exhibits uniform performance.
The lens groups L1 and L2 should not be inclined and shifted after they are assembled. However, with the conventional construction in which the first lens barrel holding the lens group L1 and the second lens barrel holding the lens group L2 are screwed into the first frame 14A and the second frame 14B, respectively, and the first frame 14A and the second frame 14B are guided respectively by the guide bar 11 and the guide rail 16, the desired performance cannot be obtained without using precision components and without assembling those components with a high accuracy. For instance, the axes of the threaded parts formed on the outer walls of the first and second lens barrels 17 and 18 should be established accurately with respect to the holes which are cut in the first and second frames 14A and 14B to receive the guide bar, the guide bar 11 and the guide rail 16 should be strictly parallel and the positional accuracy of the rollers 15 rolling on the guide rail 16 with respect to the threaded parts should also be maintained high. However, it is considerably difficult to do so, and these requirements cannot be met without a high manufacturing cost and much maintenance.